Blender 2.8 startup function working in addon
Ok figured it out, addons get loaded after the startup.blend is loaded, so load_post wouldn't work. Also in 2.8 scene_update_post has been replaced with depsgraph_update_post. (Which is much better imo) So i run the search and then delete the handler:

bl_info = {
    "name": "test",
    "author": "test",
    "version": (1, 0),
    "blender": (2, 80, 0),
    "description": "Auto-fills addon search field",
    "category": "3D View"}

import bpy
from bpy.app.handlers import persistent


@persistent
def addon_search(scene):
    bpy.data.window_managers["WinMan"].addon_search = "editor"
    bpy.app.handlers.depsgraph_update_post.remove(addon_search)


def register():
    bpy.app.handlers.depsgraph_update_post.append(addon_search)

def unregister():
    bpy.app.handlers.depsgraph_update_post.remove(addon_search)

if __name__ == "__main__":
    register()